The present invention relates to a method of manufacturing dental prostheses, such as an inlay, crown, base, implant, and upper part of implant, from precious metals or nonprecious metals by pressure casting. The present invention also relates to an apparatus for casting dental prostheses.
If a person loses a part or whole of the teeth as a result of caries (tooth decay), periodontal disease or the like, the person suffers not only functional declination in speech and chewing and/or a change in the facial appearance, but also the health of the whole body is influenced badly thereby. It is therefore important to undergo a treatment as soon as possible to restore the missing tooth (or teeth). According to one of the known restoration methods, a metallic casting is put in place of the missing part of the teeth. Dental prostheses for restoring missing parts of teeth, however, cannot be mass-produced because there is a significant individual difference in the shape of teeth, and the size and shape of the missing part differs depending on the case. Therefore, it is necessary to manufacture a prosthesis having a particular shape depending on the case of each patient. Also, it is necessary to manufacture the prosthesis with a high degree of accuracy in order to provide a correct occlusion. Thus, in the field of dental casting, the lost wax process, which is known for providing a high degree of accuracy of casting, is generally used for obtaining castings that meet the above demands.
FIG. 6 is a flow chart showing the steps of dental casting according to the lost wax process. Referring to this FIG. 6, the steps of manufacturing a prosthesis used for dental treatment is described. At first, a dentist takes a negative impression model of the mouth and teeth around the object part of a patient (Step S1). A dental technician pours modeling material, such as gypsum, into the negative impression, and solidifies the material to produce a positive model (Step S2). The dental technician forms a casting model of the object part such as an inlay or crown using wax or resin on the positive model (Step S3). A sprue wire for forming a sprue runner is attached to an appropriate part of the casting model with wax or the like (Step S4). After that, the casting model is detached from the positive model, and the free end of the sprue wire is stabbed on a crucible former (Step S5).
FIG. 7 is a front view of a casting model mounted on a commonly used crucible former. The crucible former 60 has a conical base 61 formed at its center, and a hole 62 for inserting the sprue wire 64 is formed on the top of the conical base 61. The hole 62 is filled with softened wax and the free end of the sprue wire 64 (to which the casting model 63 is attached) is inserted in the soft wax. When the wax solidifies, the casting model 63 is fixed on the top of the conical base 61 with the sprue wire 64.
A metallic cylinder (not shown) is fit on the crucible former 60 so that the casting model 63 is surrounded by the cylinder, and investment material such as gypsum or phosphate is poured into the cylinder to conceal the casting model 63 (Step S6). After the investment material is solidified, the crucible former 60 is removed, and the investment material is heated (Step S7). By heating, the wax inside is burned off, leaving a cavity corresponding to the sprue wire 64 and the casting model 63. Thus, a mold is obtained.
Heating the mold to a preset temperature, molten metal is poured into a reservoir at the top of the mold, which is a conical depression having a shape corresponding to the conical base of the crucible former, and the molten metal flows into the cavity through the sprue runner (Step S8). After the poured metal has cooled down and solidified, the mold is broken to take out the casting inside (Step S9). Then, unnecessary parts such as fringe metals along the sprue runner is removed from the casting, and after-treatments such as sanding of the surface is carried out (Step S10). Thus, a prosthesis is completed.
For assisting molten metal to flow smoothly into the cavity in Step S8, one of the following three methods is generally used. The first is the centrifugal casting wherein the mold is revolved around an axis so that the molten metal is pressurized into the mold by the effect of the centrifugal force. The second is the pressure casting wherein the molten metal is at first poured into the reservoir under a vacuum, and the pressure is then increased so that the molten metal is forced into the cavity due to the pressure difference between the cavity and the outside (this process is referred to as xe2x80x9cpressurizing operationxe2x80x9d in this specification). The third is the vacuum casting wherein the molten metal poured into the reservoir is introduced into the cavity by evacuating the cavity from the other side.
Japanese Unexamined Patent Publication Nos. H06-126422 and H07-132364 disclose conventional casting apparatuses utilizing the pressure casting. The apparatus disclosed therein has a casting chamber in which a crucible and a mold are oppositely positioned across a horizontal axis so that the top of the crucible and the reservoir of the mold face each other. The casting chamber is rotatable about the horizontal axis to turn the posture of the casting chamber, or the relative position of the crucible and the mold, upside-down. At first the casting chamber is held in a position where the open top of the crucible is directed upwards, alloy ingots are put in the crucible, and the crucible is heated to melt the alloy. The mold located above the crucible is heated to a temperature of about 800-900 C (degrees Celsius), and the casting chamber is evacuated with a vacuum pump. At a preset timing when the pressure in the cavity inside the mold decreases adequately, the casting chamber is turned upside-down. The open top of the crucible is now directed downwards, and the molten metal in the crucible is poured into the reservoir of the mold. The molten metal closes the mouth of the sprue runner in an airtight manner. After that, the evacuation of the casting chamber is stopped, and the pressure in the casting chamber is increased by supplying gas such as pressurized air or inert gas, while the pressure in the cavity of the mold is maintained low. Due to the pressure difference, the molten metal is forced to flow through the sprue runner into the cavity of the mold. Thus, the pouring of the molten metal is completed.
In the above-described pressure casting process, if the pressurizing operation is started before an appropriate amount of molten metal is poured in the reservoir, the gas used for increasing the pressure in the chamber intrudes into the cavity, which results in defects in the casting. For preventing this, it is desirable to start the pressurizing operation after an adequate amount of molten metal is poured in the reservoir.
When an alloy having a high melting point (usually over 1000 C), such as nickel-chromium alloy or cobalt-chromium alloy, is used as the casting material, the following problem must be considered. Even when a mold is made of an investment material durable to high temperature casting (e.g. material of the phosphate group), the highest allowable temperature is about 900 C. This means that, in casting, the temperature of the mold is normally lower than the melting point of the molten metal. Therefore, it is probable that the molten metal solidifies before the pressurizing operation starts since the temperature of the molten metal decreases rapidly after the molten metal is poured into the mold. Even if the molten metal does not solidify, casting defects due to an inadequate pouring is likely to occur since a lowered fluidity of the molten metal prevents it from flowing smoothly.
When the temperature of the molten metal is set higher, the time required for the solidification of the molten metal poured into the mold becomes longer, and the casting workability is enhanced. When, however, the temperature is too high, the molten metal reacts with the investment material of the mold, and the surface of the casting becomes rough. As a result, the time required for the after-treatments such as sanding increases, and the working efficiency deteriorates. Also, too high a temperature of the molten metal often causes undesirable effects on the properties such as the hardness or toughness of the casted object.
For addressing the above-described problems, an object of the present invention is to propose a pressure casting method which provides high casting workability even when a metal having a high melting point is used. Also, the present invention proposes an apparatus for casting a dental prosthesis according to the method of the present invention.
Thus, the present invention proposes a method of casting a dental prosthesis by pressure casting using an airtight chamber containing a crucible and a mold with an open top of the crucible and a mouth of a sprue runner of the mold facing each other across a rotating axis of the airtight chamber, the method including the steps of:
putting the airtight chamber at a position where the crucible with a quantity of metallic material is at the bottom and the mold is at the top;
heating the crucible to melt the quantity of metallic material to produce a quantity of molten metal; and
rotating the airtight chamber around the rotating axis until the airtight chamber is substantially reversed,
which is characterized in that:
the pressure in the airtight chamber is maintained low until a preset time point before the rotation of the airtight chamber is completed, and a pressure gas is supplied in the airtight chamber at the preset time point.
Also, the present invention proposes an apparatus for casting a dental prosthesis by pressure casting, including:
an airtight chamber containing a crucible and a mold with an open top of the crucible and a mouth of a sprue runner of the mold facing each other across a rotating axis of the airtight chamber;
a heater for heating the crucible;
a rotating mechanism for rotating the airtight chamber around the rotating axis;
a position sensor for detecting a preset angular position of the airtight chamber between a normal position where the crucible is at the bottom and the mold is at the top and a reversed position where the mold is at the bottom and the crucible is at the top;
a delay time storing means for storing an externally given delay time; and
a pressure controller for maintaining a low pressure in the airtight chamber until a second time point which is the delay time after a first time point when the position sensor detects the preset angular position of the airtight chamber and for supplying a pressure gas in the airtight chamber at the second time point.
In conventional pressure casting methods, the pressure gas is supplied into the chamber when or after the chamber arrives at the reversed position. In the pressure casting method according to the present invention, on the other hand, the pressure gas is supplied before the rotation of the airtight chamber is completed. In other word, the gas is supplied during the rotation of the chamber at a time point when the chamber arrives at an angular position preset between the normal position and the reversed position.
In general, a high melting point metal has high fluidity (or low viscosity) in the molten state. Therefore, when the chamber comes to an angular position of about 120 degrees from the normal position (or 60 degrees to the reversed position), the molten metal starts flowing out of the inclined crucible and is poured into the reservoir of the mold. When the molten metal completely closes the mouth of the sprue runner, the pressure gas is supplied into the chamber. As the pressure increases, the molten metal is pressed through the sprue runner into the cavity inside the mold due to the pressure difference between the cavity and the outside of the mold. The further the chamber rotates, the more the molten metal is poured into the mold. Thus, the molten metal is continuously supplied into the sprue runner following to the molten metal already existing in the sprue runner. Since the molten metal poured into the mold is promptly pressed into the cavity, the molten metal is prevented from solidifying at the mouth of or in the sprue runner. Since the pressurizing operation is started after the molten metal completely closed the mouth of the sprue runner, the pressure gas is prevented from intruding into the sprue runner.
The fluidity of a molten metal depends on the kind of the metal, temperature of the metal, and other factors. Therefore, it is preferable to determine an appropriate timing for starting the pressurizing operation according to the above factors. Thus, the apparatus according to the present invention is provided with the means for storing the delay time for the operator to determine an appropriate delay time according to the above factors, whereby the casting workability is assuredly maintained at high level.
As described above, by using a pressure casting method or apparatus according to the present invention, the molten metal poured into the reservoir of the mold smoothly flows through the sprue runner into the cavity since the pressurizing operation is started at an appropriate timing before the molten metal solidifies. Therefore, casting defects due to inadequate casting are prevented, and the manufacturing efficiency of dental prosthesis is improved.